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  <title>WebGL - 2D - DrawImage with MatrixStack rotation point in center of image</title>
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  <div id="info">
    <a href="https://webglfundamentals.org/webgl/lessons/zh_cn/webgl-2d-matrix-stack.html" target="_blank">WebGL -
      二维矩阵栈</a>
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  <canvas id="canvas"></canvas>
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<!-- vertex shader -->
<script id="drawImage-vertex-shader" type="x-shader/x-vertex">
attribute vec4 a_position;
attribute vec2 a_texcoord;

uniform mat4 u_matrix;
uniform mat4 u_textureMatrix;

varying vec2 v_texcoord;

void main() {
   gl_Position = u_matrix * a_position;
   v_texcoord = (u_textureMatrix * vec4(a_texcoord, 0, 1)).xy;
}
</script>
<script id="drawImage-fragment-shader" type="x-shader/x-fragment">
precision mediump float;

varying vec2 v_texcoord;

uniform sampler2D texture;

void main() {
   gl_FragColor = texture2D(texture, v_texcoord);
}
</script>
<!--
for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
See http://webglfundamentals.org/webgl/lessons/webgl-boilerplate.html
and http://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
for webgl-utils, m3, m4, and webgl-lessons-ui.
-->
<script src="../../resources/webgl-utils.js"></script>
<script src="../../resources/m4.js"></script>
<script>
  "use strict";

  function main() {
    // Get A WebGL context
    /** @type {HTMLCanvasElement} */
    var canvas = document.getElementById("canvas");
    var gl = canvas.getContext("webgl");
    if (!gl) {
      return;
    }

    var matrixStack = new MatrixStack();

    // setup GLSL program
    var program = webglUtils.createProgramFromScripts(gl, ["drawImage-vertex-shader", "drawImage-fragment-shader"]);

    // look up where the vertex data needs to go.
    var positionLocation = gl.getAttribLocation(program, "a_position");
    var texcoordLocation = gl.getAttribLocation(program, "a_texcoord");

    // lookup uniforms
    var matrixLocation = gl.getUniformLocation(program, "u_matrix");
    var textureMatrixLocation = gl.getUniformLocation(program, "u_textureMatrix");
    var textureLocation = gl.getUniformLocation(program, "u_texture");

    // Create a buffer.
    var positionBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

    // Put a unit quad in the buffer
    var positions = [
      0, 0,
      0, 1,
      1, 0,
      1, 0,
      0, 1,
      1, 1,
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

    // Create a buffer for texture coords
    var texcoordBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBuffer);

    // Put texcoords in the buffer
    var texcoords = [
      0, 0,
      0, 1,
      1, 0,
      1, 0,
      0, 1,
      1, 1,
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(texcoords), gl.STATIC_DRAW);

    // creates a texture info { width: w, height: h, texture: tex }
    // The texture will start with 1x1 pixels and be updated
    // when the image has loaded
    function loadImageAndCreateTextureInfo(url) {
      var tex = gl.createTexture();
      gl.bindTexture(gl.TEXTURE_2D, tex);
      // Fill the texture with a 1x1 blue pixel.
      gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE,
        new Uint8Array([0, 0, 255, 255]));

      // let's assume all images are not a power of 2
      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
      gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);

      var textureInfo = {
        width: 1,   // we don't know the size until it loads
        height: 1,
        texture: tex,
      };
      var img = new Image();
      img.addEventListener('load', function () {
        textureInfo.width = img.width;
        textureInfo.height = img.height;

        gl.bindTexture(gl.TEXTURE_2D, textureInfo.texture);
        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, img);
      });
      img.src = url;

      return textureInfo;
    }

    var textureInfo = loadImageAndCreateTextureInfo('../../resources/star.jpg');

    function draw(time) {
      webglUtils.resizeCanvasToDisplaySize(gl.canvas);

      // Tell WebGL how to convert from clip space to pixels
      gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

      gl.clear(gl.COLOR_BUFFER_BIT);

      matrixStack.save();
      matrixStack.translate(gl.canvas.width / 2, gl.canvas.height / 2);
      matrixStack.rotateZ(time);
      matrixStack.translate(textureInfo.width / -2, textureInfo.height / -2);

      drawImage(
        textureInfo.texture,
        textureInfo.width,
        textureInfo.height,
        0, 0);

      matrixStack.restore();
    }

    var then = 0;
    function render(time) {
      time *= 0.001;
      var now = time;
      var deltaTime = Math.min(0.1, now - then);
      then = now;

      draw(time);
      requestAnimationFrame(render);
    }
    requestAnimationFrame(render);

    // Unlike images, textures do not have a width and height associated
    // with them so we'll pass in the width and height of the texture
    function drawImage(
      tex, texWidth, texHeight,
      srcX, srcY, srcWidth, srcHeight,
      dstX, dstY, dstWidth, dstHeight) {
      if (dstX === undefined) {
        dstX = srcX;
      }
      if (dstY === undefined) {
        dstY = srcY;
      }
      if (srcWidth === undefined) {
        srcWidth = texWidth;
      }
      if (srcHeight === undefined) {
        srcHeight = texHeight;
      }
      if (dstWidth === undefined) {
        dstWidth = srcWidth;
      }
      if (dstHeight === undefined) {
        dstHeight = srcHeight;
      }

      gl.bindTexture(gl.TEXTURE_2D, tex);

      // Tell WebGL to use our shader program pair
      gl.useProgram(program);

      // Setup the attributes to pull data from our buffers
      gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
      gl.enableVertexAttribArray(positionLocation);
      gl.vertexAttribPointer(positionLocation, 2, gl.FLOAT, false, 0, 0);
      gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBuffer);
      gl.enableVertexAttribArray(texcoordLocation);
      gl.vertexAttribPointer(texcoordLocation, 2, gl.FLOAT, false, 0, 0);

      // this matirx will convert from pixels to clip space
      var matrix = m4.orthographic(0, gl.canvas.width, gl.canvas.height, 0, -1, 1);

      // this matrix moves the origin to the one represented by
      // the current matrix stack.
      matrix = m4.multiply(matrix, matrixStack.getCurrentMatrix());

      // this matrix will translate our quad to dstX, dstY
      matrix = m4.translate(matrix, dstX, dstY, 0);

      // this matrix will scale our 1 unit quad
      // from 1 unit to texWidth, texHeight units
      matrix = m4.scale(matrix, dstWidth, dstHeight, 1);

      // Set the matrix.
      gl.uniformMatrix4fv(matrixLocation, false, matrix);

      // Because texture coordinates go from 0 to 1
      // and because our texture coordinates are already a unit quad
      // we can select an area of the texture by scaling the unit quad
      // down
      var texMatrix = m4.translation(srcX / texWidth, srcY / texHeight, 0);
      texMatrix = m4.scale(texMatrix, srcWidth / texWidth, srcHeight / texHeight, 1);

      // Set the texture matrix.
      gl.uniformMatrix4fv(textureMatrixLocation, false, texMatrix);

      // Tell the shader to get the texture from texture unit 0
      gl.uniform1i(textureLocation, 0);

      // draw the quad (2 triangles, 6 vertices)
      gl.drawArrays(gl.TRIANGLES, 0, 6);
    }

  }

  function MatrixStack() {
    this.stack = [];

    // since the stack is empty this will put an initial matrix in it
    this.restore();
  }

  // Pops the top of the stack restoring the previously saved matrix
  MatrixStack.prototype.restore = function () {
    this.stack.pop();
    // Never let the stack be totally empty
    if (this.stack.length < 1) {
      this.stack[0] = m4.identity();
    }
  };

  // Pushes a copy of the current matrix on the stack
  MatrixStack.prototype.save = function () {
    this.stack.push(this.getCurrentMatrix());
  };

  // Gets a copy of the current matrix (top of the stack)
  MatrixStack.prototype.getCurrentMatrix = function () {
    return this.stack[this.stack.length - 1].slice();
  };

  // Lets us set the current matrix
  MatrixStack.prototype.setCurrentMatrix = function (m) {
    this.stack[this.stack.length - 1] = m;
    return m;
  };

  // Translates the current matrix
  MatrixStack.prototype.translate = function (x, y, z) {
    if (z === undefined) {
      z = 0;
    }
    var m = this.getCurrentMatrix();
    this.setCurrentMatrix(m4.translate(m, x, y, z));
  };

  // Rotates the current matrix around Z
  MatrixStack.prototype.rotateZ = function (angleInRadians) {
    var m = this.getCurrentMatrix();
    this.setCurrentMatrix(m4.zRotate(m, angleInRadians));
  };

  // Scales the current matrix
  MatrixStack.prototype.scale = function (x, y, z) {
    if (z === undefined) {
      z = 1;
    }
    var m = this.getCurrentMatrix();
    this.setCurrentMatrix(m4.scale(m, x, y, z));
  };

  main();
</script>

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